1. Field of the Invention
The present invention relates to a method for supplying ink to an inkjet printhead in an inkjet apparatus. More specifically the invention is related to a method for supplying ink to a printhead ink in an inkjet printer having a recirculation ink supply system.
2. Description of the Related Art
Nowadays a lot of printed matter is produced carrying a reproduction of a color image. A large part of these color prints in office and home environment are made using inkjet printers.
In an inkjet printer drops of ink are jetted out of nozzles of an inkjet printhead towards a receiving layer which may be e.g. specially coated paper.
Usually an inkjet print head has an array of nozzles, each nozzle jetting ink to a different location possibly at the same time.
The ink is jetted out of the nozzles by use of e.g. thermal or piezoelectric actuators creating a pressure wave.
It is normally the intention that the size of the droplets can be kept constant or that there is a good control of the droplet size in printers capable of recording variable droplet sizes.
One of the major parameters to ensure a constant drop size is that ink pressure at the printhead is stable and within a certain range suitable for the printhead used.
The pressure can be kept constant using several methods:                For small inkjet printers often a negative pressure generating member is present in the ink reservoir mounted on the shuttle carrying the printhead.        In larger printers and industrial inkjet printers an ink tank is often equipped with a system regulating and stabilizing the pressure in the tank by directly controlling the ink pressure or the pressure of the air (atmosphere) above the ink.        
In the PCT application PCT/EP2005/056809 an ink supply system, depicted in FIG. 1, for an ink jet printer is disclosed, wherein one or more ink jet print heads 1 receive ink from an ink supply tank 2. The printhead is of a throughflow type and ink circulates further to an ink return tank 3. The ink is pumped back to the ink supply tank 2 by a circulation pump 4 thereby passing trough a degassing unit 5. If additional ink is required in this circuit, it is added from main ink tank 6 by the ink feed pump 7.
In FIG. 1 the circulation through the printhead is determined only by the pressure difference between the ink supply tank 2 and the ink return tank 3. This can be generated by keeping an exact ink level difference between the ink level in the supply tank 2, remaining at a higher ink level than the level in the return tank 3, as shown in FIG. 1, or by providing a regulation system for providing regulated pressures in the supply and return tank. Another possibility is that the ink supply tank is mounted physically higher than the return tank whereby a level difference can be guaranteed.
As the “hydrostatic” or regulated pressure in the supply tank is higher that of the return tank, the ink supply tank 2 can be considered to be part of the “high” pressure section of the ink feed system while the ink return tank 3 is part of the low pressure section of the ink supply system.
As the ink flows continuously from the high pressure supply tank 2 to the low pressure return tank 3 the ink has to be re-pumped from the low pressure section to the high pressure section of the ink supply system by the circulation pump 4.
A disadvantage of this ink supply system is that gas bubbles may be created in the ink due to cavitation phenomenae in the circulation pump 4. If gas bubbles enter the print head 1, nozzles may be blocked, resulting in unreliable operation of the printer. The pump 4 may also introduce sudden level differences or pressure changes in the high pressure section.
It is clear that there is need for a method for pressurizing the ink in an inkjet printer during feeding or recirculation of the ink without the mentioned drawbacks caused by a pump.